Biomedical implants have actively been in use for stimulation and monitoring internal vital signs. Implanted sensors, drug delivery devices, neuro-stimulator and endoscopes are some of the main devices currently used for medical applications. The power requirement of these devices vary with their applications and can range from a few microwatts to hundreds of milliwatts. Because of power requirements and implant size, few devices rely on implanted batteries and most of applications use wireless power transfer to operate the implanted device or to recharge the implanted battery.
Inductive coupling between an external source and an implanted device is a popular technique for wireless power transfer to implanted devices. Traditional inductive coupling based power transfer systems use two coils in which power is transferred from external (driver) coil to implant (load) coil. The power transfer efficiency strongly depends on magnetic coupling between the coils which is governed by factors such as size, structure, physical spacing and relative location of the coils and their electric properties, such as a quality factor (Q-factor). For example, magnetic coupling between an external coil and an implant coil is drastically reduced with increase in coil spacing and hence causes rapid change in power transfer efficiency with coil misalignment.
To optimize the power link performance, electric models of 2-coil based systems have been used to identify the effect of coil parameters and coupling on link efficiency. Using low resistance wire, high unloaded Q-factor coil can be achieved, but due to use of finite source resistance of a driver and high impedance load, a loaded Q-factor can only achieve moderate values. Hence there are limits on the maximum achievable power transfer efficiency (40%) under these design constraints.
Power transfer efficiency is a commonly used metric for improvement to reduce the required power by an external source for a desired power requirement of implanted devices. For telemetry applications, performance is generally measured based on power transfer efficiency as well as on the available voltage gain and data bandwidth over the power link. Performance variations during the operation of a device are one of the main challenges for a 2-coil based system. For example, implanted coils may undergo relative motion with respect to external coils during the operation of device which causes variation in mutual coupling between an external coil and an implanted coil. Additionally, based on the power requirement of the implanted device, effective load resistance may vary and cause variation in the Q-factor of a receiving inductive unit. Hence, to ensure stable link performance, a design should have high tolerance with change in operating distance, coil misalignment, and device operation mode.